Field of the Invention
The present invention concerns a method to generate image data (in particular magnetic resonance exposures) of a subject moving inside a body, for example an organ, parts of an organ or an arbitrary target structure inside a human or animal. Moreover, the invention concerns an image data generation device to generate image data of a subject moving inside a body, as well as a magnetic resonance system with such an image data generation device.
Description of the Prior Art
The generation of image data representing the inside of the human or animal body for a later diagnosis is a standard component of many medical examinations. Various modalities are known (for example magnetic resonance tomography or computed tomography) for the generation of image data or and for the acquisition of the required raw data from which the image data can be reconstructed.
In computed tomography, raw projection data are typically generated by x-ray radiation being radiated through the body from different directions, and detection thereof with detector devices. The image data can then be reconstructed from the raw projection data.
In order to obtain image data (magnetic resonance exposures) from a region of the inside of the body of an examination subject in magnetic resonance tomography, the body or the body part to be examined must initially be exposed to an optimally homogeneous, static basic magnetic field (most often designated as a B0 field). Nuclear spins in the subject are thereby aligned parallel to the direction of the B0 field (typically designated as the z-direction). Moreover, radio-frequency pulses are radiated into the examination subject with radio-frequency antennas, the frequency of the radio-frequency pulses being at or approximately at the resonance frequency (known as the Larmor frequency) of the nuclei to be excited (normally hydrogen nuclei) in the basic magnetic field. These radio-frequency pulses are therefore designated as magnetic resonance radio-frequency pulses in the following. The magnetic flux density of these radio-frequency pulses is typically designated with B1. By means of these radio-frequency pulses, nuclear spins of the atoms in the examination subject are excited such that they are deflected by an amount known as an “excitation flip angle” (generally shortened to “flip angle”) out of their steady state (parallel to the basic magnetic field B0). The nuclear spins then initially precess around the z-direction and relax again bit by bit. The in-phase revolution of the microscopic spins around the precession cone can be viewed as a macroscopic nuclear magnetization in the x/y plane (orthogonal to the z-direction). The magnetic resonance signals generated in this relaxation of the nuclear magnetization are acquired as raw data by radio-frequency reception antennas. Magnetic resonance images of the examination subject are reconstructed on the basis of the acquired raw data. Spatial coding of the magnetic resonance signals takes place with the use of rapidly switched (activated) gradient magnetic fields that are superimposed on the basic magnetic field during the emission of the magnetic resonance radio-frequency pulses and/or the acquisition of the raw data. In the data acquisition, the raw data are initially associated with frequency domain (known as k-space). The reconstruction of the image data then takes place by means of a Fourier transformation of the raw data of k-space into the image data domain (spatial domain).
Particularly when image data of a complete organ should be generated (i.e. when either volume data or a number of densely adjoining slice images should be acquired that cover the complete region in which the organ is located) a longer measurement time is required. This is a problem if a moving organ is being examined (the heart, for example). The heart not only performs its own cyclical movements due to the heart beat, but also is additionally subject to movement due to the breathing of the patient. If raw data were simply acquired during the different movement cycles, and image data were then reconstructed from this raw data, this would lead to significant movement artifacts (for example smearing in the images), such that the generated images could often not be used for a reasonable diagnosis.
One possibility to avoid such artifacts is to acquire raw data only in a defined movement phase of the organ (heart), for example to trigger the raw data acquisition in a suitable manner. For example, with regard to the heart movement it is possible to implement such a triggering at a defined heart phase with the use of an EKG. The data are preferably acquired in the diastolic phase, since in this phase the heart movement is minimal for a relatively long time. This diastolic phase lasts approximately 100 ms. In many acquisition methods (in particular in the acquisition of the complete volume or a number of densely placed slices that cover the heart), a duration of 100 ms is not sufficient to acquire all raw data. This particularly applies in the acquisition of raw magnetic resonance data. Therefore, raw data sets or, respectively, segments of k-space must respectively be acquired in the matching cardiac phase in order to fill k-space with the necessary raw data before the image data of the volume or the desired slices are then reconstructed in the manner described above. However, the breathing movement that is additionally superimposed on the movement of the heart can lead to the situation that the heart can be situated at different locations in the body (for example in the diastolic phases of successive cardiac cycles) depending on the current movement phase of the breathing cycle. In principle, it would naturally be possible to also monitor the breathing cycle with appropriate devices and to additionally trigger on the breathing cycle. In such a case, however, raw data would only be acquired when the desired cardiac phase and the desired breathing phase randomly occur together. This would lead to a total measurement duration that is much too long, and therefore such measurement methods are not acceptable in practice.
Currently, data known as special raw navigator data are acquired from which navigator images are then generated. Normally, with these navigator images a volume is acquired in which a breathing movement can be detected particularly well. For example, in the case of exposures, the diaphragm can be observed in the navigator exposure since the liver/lung transition is very easy to detect because the liver appears relatively light in the images (due to being filled with fluid) and the lung appears relatively dark in the images (due to being filled with air). With the use of the current breathing position or breathing phase that is determined from the navigator images, the respective raw data and/or the image data of the desired subject that are acquired immediately after the acquisition of the raw navigator data are then corrected with regard to the breathing phase. However, to implement this method the acquisition of additional raw navigator data is always required, which requires additional expenditure and in particular additional measurement time.
One possibility to forego such a navigator measurement is described by Piccini D. et al. in “Respiratory Self-Navigation for Whole-Heart Bright-Blood Coronary MRI: Methods for Robust Isolation and Automatic Segmentation of the Blood Pool” in 2012, Magnetic Resonance in Medicine, 68: 571 to 579. As described therein, in order to reconstruct a volume that encompasses the complete heart, it is proposed to enter the raw data into k-space according to a phyllotactic spiral pattern. Respective segments of k-space are thereby acquired in multiple raw data sets, with respective raw data being acquired along a line in the z-direction (thus in the direction of the longitudinal axis of the body) through the center of k-space, as well as raw data along additional lines lying parallel to the central line in the z-direction. These additional lines proceed through points within an x/y plane that is orthogonal to the z-direction. The points thus lie on a spiral-shaped path that proceeds outwardly from the center. A one-dimensional projection in the SI direction (SI=Superior-Inferior, thus along the longitudinal axis of the body or in the z-direction) can then be generated (preferably) for each raw data set or each segment, on the basis of the line proceeding through the k-space center. These projections, known as “SI projections”, represent one-dimensional image data, namely the projections of the image data of the entire acquired volume (in the FoV=Field of View; viewing area) proceeding along the z-direction onto the z-axis. The breathing position can be determined from these SI projections without navigators, and a correction of the raw data and/or image data can take place in the reconstruction. This method is promising since it means a significant saving of measurement time, but the reduction of the movement artifacts has previously still been strongly dependent on the specific patient or test subject.